The beauty or attractiveness of gemstones, and in particular diamonds, is considered to be directly related to how the stones interact with light. The market value of a polished diamond depends on its colour, cut proportions, internal clarity and weight, known as the “Four Cs”. The cut and weight are relatively easy to measure quantitatively, but colour is generally measured by eye. In the case of colourless to light yellow coloured diamonds, an analysis is made along what is commonly referred to as the GIA D to Z scale. Typically a trained assessor visually inspects the diamond, and compares it to a set of existing sample stones of known colour. In practice, the sample stones usually have a colour which corresponds to the boundary between grades—e.g. a D/E boundary stone.
This visual inspection process is difficult, time consuming, and inevitably involves some subjective judgement. It is therefore desirable to produce an instrument which can provide a repeatable, reliable, objective quantitative measurement of the colour of a diamond. Various approaches have been tried in the past. In general, what they have in common is the illumination of the diamond by visible light, and the measurement of light transmitted from the diamond. Colour analysis of this light enables a measure of the diamond's colour to be estimated.
Although the these techniques can provide a useful indication of the colour of a diamond, in practice, simple colour analysis of the light emitted from a diamond does not always correlate exactly with colour grades issued by trained assessors. It is believed that this is due to subtle differences visible by eye which can be caused by the many different path lengths which may be followed by light transmitted by a diamond or other polished gemstone. These different path lengths are caused by internal reflection from the facets of the stone. The techniques referred to above effectively measure a single light path, a region corresponding to a collection of similar length light paths or an average of some or all light paths. A human eye, by contrast, will see an image of the whole gemstone, and can simultaneously see the colour across the stone, and variations in the apparent colour caused by differences in path length of light traversing the stone.
To be able to detect subtle variations in colour, instruments require careful calibration and a stable operating environment. This affects repeatability and reproducibility and limits the sensitivity with which colour can be measured. Furthermore, subtle variations in the hue of a diamond can have a dramatic effect on its price. For example, a diamond with a yellow hue is significantly more valuable than a similar diamond with a brown hue. Existing techniques struggle to detect these hue changes in near colourless diamonds. In addition, existing techniques measure relative to a standard or a patch of known colour. The standard or colour patch must be cleaned and maintained.